Detailed studies are proposed for the following electron paramagnetic resonance (EPR) experiments on human nitrosylhemoglobin (HbNO) single crystals. Each type of experiment has been demonstrated in preliminary experiments in our laboratory. 1. A study of the different rates of NO uptake by the Alpha- and Beta-subunits in human oxyhemoglobin HbO2 crystals. 2. A study of heme dynamics and disorder, which differ measurably in the Alpha- and Beta-subunits, by means of the temperature and microwave power dependence of the EPR line shapes. 3. Measurment of the wavelength dependence and yield of the photolysis of nitrosylhemoglobin at 4.2 percent K by means of the disappearance of the EPR spectrum. 4. A study of the measurably different kinetics of NO recombination with the five-coordinate Alpha- and Beta-subunits of the hemoglobin after photolysis.